With continual improvements of a scientific and technological level, an Organic Light-Emitting Diode (OLED), as a light-emitting device, has been known more and more by people and has been applied to a display apparatus of high performance widely. The OLED has a wide application prospect because of its advantages such as simple manufacture process, high luminous brightness, quick response speed, low cost, and appropriate operating temperature, etc.
Depending on different driving manners, the OLED may be classified as the Passive Matrix Organic Light-Emitting Diode (PMOLED) and the Active Matrix Organic Light-Emitting Diode (AMOLED). The Passive Matrix driving has a simple process and a low cost, but it requires a shorter driving time for a single pixel with the increasing of the size of a display apparatus thereby the transient current need be increased and the power consumption increases. Also, the increasing of the transient current may lead that the voltage drop on the scan lines and the data lines becomes larger, and the required operating voltage required is raised, resulting in a decrease for the display efficiency. Therefore, many companies focus more attention on the Active Matrix driving manner.
As a common pixel driving circuit structure in the Active Matrix driving manner, as shown in FIG. 1, the pixel driving circuit in the prior art comprises a driving transistor M1, a switch transistor M2, an organic light-emitting diode OLED and a capacitor C1. When the scan voltage is at a high level, the switch transistor M2 is turned on, and the capacitor C1 is charged by a data voltage signal Vdata of the high level. When the scan voltage is at a low level, the switch transistor M2 is turned off, the capacitor C1 is discharged and the driving transistor M1 is kept in a turn-on state. Therefore, the driving transistor M1 is in a saturated turn-on state during a normal operating process. That is, the OLED is in a constant current control process during the whole operating period. The driving current of the light-emitting diode OLED satisfies an equation as follows, according to the calculation equation for the leakage current of the transistor:
            I      OLED        =                            1          2                ⁢                              μ            n                    ·          Cox          ·                      W            L                    ·                                    (                              Vgs                -                Vthn                            )                        2                              =                        1          2                ⁢                              μ            n                    ·          Cox          ·                      W            L                    ·                                    (                              Vg                -                Vs                -                Vthn                            )                        2                                ,
where μn is the carrier mobility, COX is the value of the insulating film capacitance at the gate per unit area,
  W  Lis the width-length ratio of the driving transistor M1, and (Vgs−Vthn) is the over-driving voltage of the driving transistor M1. Herein, Vgs is the voltage difference between the gate and the source of the driving transistor M1, and Vthn is the threshold voltage of the driving transistor M1. Further, Vgs=Vg−Vs=Vdata−(VOLED+ARVSS), Vdata is the data voltage, VOLED is the operating voltage of the OLED, and ARVSS is the common ground terminal voltage. It can be seen that an effect of controlling the constant current for driving the OLED may be achieved by controlling the data voltage Vdata, and since the light-emitting brightness of the OLED is directly proportional to the constant current, the purpose of changing the light-emitting brightness of the OLED may be achieved by controlling the data voltage Vdata.
However, during the development process, the inventors find that the prior arts have at least the following disadvantages: the threshold voltages Vthn of each driving transistor in the pixel driving circuit in the prior art is different because of the process limitations or a shift phenomenon generated under a long time of pressure and a high temperature, resulting in that the over-driving voltage of the respective driving transistors are not consistent, and a non-uniformity of the threshold voltages would finally lead to differences in the display brightness of the display apparatus.